What Google reads in it
Each entry is a url element with the page address in loc and, optionally, a lastmod date. Google's documentation says it ignores the priority and changefreq values, and uses lastmod only when it is consistently and verifiably accurate. One sitemap can hold up to 50,000 URLs or 50MB uncompressed; a bigger site splits them and lists the files in a sitemap index.
Do you need one?
Google says a site might not need a sitemap if it is small, about 500 pages or fewer that matter for search, and every important page can be reached by following links from the home page. You are more likely to need one when the site is large, new with few links from other sites, or has a lot of video, images or news.
A sitemap is a hint. Listing a URL doesn't guarantee it gets crawled or indexed.
Keeping it useful
List only pages you want in search results: no redirects, no noindex pages, no duplicates of a canonical URL. Update lastmod only when the content really changes, then submit the sitemap in Search Console or add it to robots.txt.